This invention relates to wafer handling methods and apparatus.
This invention has particular application to methods and apparatus for insuring centering of a wafer at a work station and to methods and apparatus for heating a wafer to bake off fluids such as, for example, solvents used in applying a photoresist to the wafer or liquids used in washing or cleaning the wafer.
In the processing of silicon wafers used in the manufacture of semiconductors there are a number of operations which require fluids to be removed from the wafer before subsequent processing steps can be carried out.
For example, after a layer of photoresist has been centrifugally spun onto the wafer, the solvents in the photoresist are driven out by baking. In automated equipment for processing wafers this bake operation, referred to as a "soft bake", is often performed in an oven by radiation baking.
Another application which requires baking is the removal of unwanted portions of the photoresist material after the patterns are exposed onto the photoresist. The unwanted material is washed away, and after this operation the wafer is baked in a "hard bake" operation to make the photoresist that remains impervious to etchents.
There is also a "dehydration bake" operation. This is used to remove residual moisture that remains on the wafer after a wafer is washed and cleaned by water or detergent.
There has been a trend to go to conduction baking (putting the wafer on a hot plate) to bake off the fluid rather than to use radiation baking in ovens. The machinery used for hot plate baking can be made smaller than the radiation oven, and floor space is often at a premium in wafer processing production.
Controlling the temperature rise time to the equilibrium or bake out temperature of the wafer with a hot plate can present problems. If the rise time is too quick, bubbles may form in the photoresist material; and if the rise time is too slow, the time for the baking operation is unduly extended.
It is an important object of the present invention to use a hot plate to bake a wafer in a way that provides close control of the wafer temperature at every point in the heating cycle and that also permits flexibility in the way the wafer is brought up to the equilibrium or bake out temperature while minimizing the total time required for the baking operation.
It is a specific object of the present invention to heat the wafer by a proximity baking technique in which the amount of heat transferred to the wafer and the rate at which the heat is transferred to the wafer are determined by the distance between the wafer and the hot plate.
It is another specific object of the present invention to run the hot plate at a constant temperature higher than the highest temperature to which the wafer is to be heated, so that the hot plate, in effect, serves as an overdriven source and the temperature control of the wafer is obtained either by proximity programmed or by measured temperature responsive servo control of the distance between the wafer and the hot plate.
It is another object of the present invention to heat the wafer by method and apparatus which do not require contact of the wafer with either the hot plate or a pyrometer for measuring the temperature of the wafer.
For most wafer processing operations at a work station, accurate centering of the wafer with respect to the center of the work station is important. For example, centering is critical in spinning operations.
It is another important object of the present invention to locate a pair of sensors at the work station in locations which detect the presence of the wafer at the work station area and which co-act with the wafer transport mechanism and controller to insure that the wafer is properly centered before the wafer is lifted from the wafer transport mechanism at the work station for the start of a processing operation.
Using two sensors to make wafer centering independent of the position of the flat on the wafer is another object of the present invention.